1. Field of the Invention
The present invention relates generally to structural improvements in bicycle frames, and it relates more particularly to a bicycle frame seat stay assembly providing structural features allowing increased clearance between the rear wheel at down stays in the area of the rear tire; increased brake stiffness, and frame down stay compliance while maintaining lateral stiffness.
2. Description of the Prior Art
Most conventional bicycle frames include a generally vertical seat tube having a pair of generally straight diagonally downwardly extending "seat stays" connected directly or indirectly at their upper ends to the seat tube and terminating at their lower ends in "drop outs" in the area of the rear wheel axle support. The seat stays are essential to give the rear wheel adequate support under the load of the rider as well as additional dynamic loads imposed on the rear wheel during riding conditions.
One popular version of seat stays is a "mono-stay" or "wishbone" which consists of two seat stays which run from the drop outs to above the wheel and then curve in towards each other. One larger tube then links the two seat stays to the back of the seat tube. Seat stays affect many parameters of bike performance, including braking, tire clearance and rear frame stiffness.
Virtually all current mountain bikes have cantilever type brakes which are mounted to bosses on the seat stays in the area where the rear bicycle tire passes between the stays. These brakes comprise two separate brake arms linked by a cable running to a handlebar mounted brake lever. The brakes have pads which engage opposite sides of the tire rim and by mechanical advantage they are designed to apply a considerable force to the rim.
When the brakes are applied, the seat stays of a conventional bicycle frame can be observed to bow outwardly to counteract the braking force. This bowing generally hampers bicycle performance and decreases brake efficiency. Attempts have been made to minimize bowing by having the stays locally thickened or increased in diameter in the area where the brakes are attached. However this adds weight to the frame and increases vertical stiffness of the frame which is not desirable.
Certain stiffness traits are however considered desirable, particularly in a mountain bike. The rear part of the frame should preferably be vertically compliant and laterally stiff. Vertical compliance reduces the shock felt by the rider and improves traction allowing the rear wheel to roll over obstacles rather than bounce over them. Lateral stiffness is desirable from a handling and cornering point of view. Lateral flexing or twisting of the rear wheel impairs cornering and climbing performance of the bicycle. Typically, a large section seat stay will stiffen a frame in both the lateral and vertical directions. Thus frame designers are forced to choose a section which is a compromise.
Mountain bikes are designed for off-road use, which includes travel over rocks, dirt roads, vegetation, mud, etc. It has been widely recognized that maximum tire clearance is beneficial. "Tire clearance" is the gap between the tire and nearest frame member. Maximum tire clearance is desirable for two reasons. First, in recent years there has been a trend to wider tires. Wider tires minimize the available clearance, forcing designers to provide more clearance than was previously necessary. Second, more clearance is desirable for poor weather conditions and mud. Mud or vegetation can build up on a tire and if there is insufficient clearance, this mud or vegetation can contact the frame members which may severely impair the bicycle's performance.
Seat stays are among the most difficult frame members to provide adequate clearance for. Their location, aside the rear wheel, makes them among the tubes most likely to be affected by accumulations of mud or vegetation. Moreover, certain design constraints make it difficult to increase the gap between seat stays. First, the top of the stays must join into the seat tube directly, or into a mono-stay, which then joins the seat tube. This forces the top portions of the tubes to be relatively close to each other, near the location where the tire is at its widest. Second, because of standard brake dimensions, most manufacturers mount the brake studs on the inboard regions of the seat stays to maximize the gap between seat stays. However, even with such an offset, the stays must remain relatively close together at the rim area. In addition, the location of the brake bosses to the inward part of the tube (rather than centered directly over the tube) increases the stresses in this region and results in more brake flex.